The invention relates generally to rotary lawn mowers and to cutter blades for rotary lawn mowers. The invention further relates to rotary mowers which discharge the grass clippings or which mulch the grass clippings. Still more particularly, the invention relates to rotary cutter blades which are driven at rotary speeds which produce tip speeds of between 14,000 fpm and 19,000 fpm.
In conventional rotary mowers, the rotating cutter blade not only cuts the grass but agitates and moves air. Grass discharge and bagging characteristics are dependent on the air flow characteristics of the mower. To optimize these characteristics, the cutter blade and mower housing should cooperate with each other at the nominal speed of operation.
With conventional blade shape, wherein the sail portion is formed upwardly for a length which is essentially of uniform height and parallel to the longitudinal axis of the blade, the bulk of air is pumped forwardly or tangentially ahead of the blade in a high density airstream. Since that airstream impinges or strikes the circular wall or skirt of the housing, much of its energy and velocity is lost through fluid friction, resulting in wasted power. In addition, the cutter blade continuously shears its way through that high density airstream, wasting more power. As the cutter blade shears that airstream, noise is generated. The greater the density of the airstream, the greater the noise generated. Noise is a discordant type of sound power. That power is, also, wasted.
Attention is directed to the Comer U.S. Pat. Nos. 4,129,977 issued Dec. 19, 1978 and 4,149,358 issued Apr. 17, 1979. These patents are directed to cutter blades which are intended to operate at tip speeds below 14,000 fpm, which are designed to provide predominant mechanical transport and suspension rather than pneumatic transport and suspension, and which, when operated at or above 14,000 fpm, cause mower "blow-out" with entrained air (and clippings) being driven out or around the lower edges of the mower housing rather than being channeled into a bag. In addition, operation of the Comer cutter blades at such speeds is likely to result in undue turbulence within the housing and to interfere with orderly cutting and evacuation of the housing.
By contrast to the cutter blades of U.S. Pat. Nos. 4,129,977 and 4,149,358, the cutter blade described and claimed hereinafter is designed to provide pneumatic suspension and transport and does not cause "blow-out" at tip speeds within the range of 14,000 fpm to 19,000 fpm.